In mechanical engineering, there is an increasing search to obtain materials for applications which require properties, such as high mechanical strength and high wear strength allied to a low coefficient of friction.
Nowadays, wear and corrosion problems jointly represent losses from 2% to 5% of World GDP; about 35% of the whole mechanical energy produced in the planet is lost due to lubrication deficiency and is converted in heat by friction. Apart from the energy loss, the generated heat impairs the performance of the mechanical system due to heating. Thus, maintaining a low coefficient of friction in mechanical pieces under friction is highly important, not only for energy economy, but also to enhance the durability of said pieces and of the mechanical systems in which they operate, besides contributing to environment preservation.
The manner being used to reduce wear and friction between surfaces in relative movement is to maintain these surfaces separated, interleaving a lubricating layer therebetween. Among possible lubricating ways, the hydrodynamic (fluid lubricants) is the most used. In the hydrodynamic lubrication there is formed an oil film which separates completely the surfaces in relative movement. However, it should be pointed out that the use of fluid lubricants is usually problematic, as in applications at very high or very low temperatures, in applications in which the fluid lubricant may chemically react and when the fluid lubricant may act as a contaminant. Besides, in situations of limit lubrication resulting from cycle stops, or in situations in which it is impossible to form a continuous oil film, there occurs contact between the pieces, consequently causing wear to the latter.
The dry lubrication, that is, the one using solid lubricants, is an alternative to the traditional lubrication, since it acts by the presence of a lubricating layer, which prevents the contact between the component surfaces but without presenting rupture of the formed layer.
The solid lubricants have been well accepted in problematic lubrication areas. They can be used at extreme temperatures, under high-load conditions and in a chemically reactive environment, where conventional lubricants cannot be used. Moreover, dry lubrication (solid lubricants) is an environmentally cleaner alternative.
The solid lubricant may be applied to the components of a tribological pair, in the form of films (or layers) that are deposited or generated on the surface of the components or incorporated to the volume of the material of said components, in the form of second-phase particles. When specific films or layers are applied and in case they suffer wear, there occurs the metal-metal contact and the consequent and rapid wear of the unprotected confronting surfaces and of the relatively movable components. In these solutions in which films or layers are applied, it should be further considered the difficulty in replacing the lubricant, as well as the oxidation and degradation of the latter.
Thus, a more adequate solution which allows increasing the lifetime of the material, that is, of the components, is to incorporate the solid lubricant into the volume of the material constitutive of the component, so as to form the structure of the component in a composite material of low coefficient of friction. This is possible through the technology of processing materials from powders, that is, by the conformation of a powder mixture by compaction, including pressing, rolling, extrusion and others, or also by injection molding, followed by sintering, in order to obtain a continuous composite material, usually already in the final geometry and dimensions (finished product) or in geometry and dimensions close to the final ones (semi-finished product).
Self-lubricating mechanical components (powder metallurgy products) presenting low coefficient of friction, such as sintered self-lubricating bushings, produced by powder metallurgy from composite materials and comprising a particulate precursor which forms the structural matrix of the piece, and a particulate solid lubricant to be incorporated into the structural matrix of the piece, have been used in diverse household appliances and small equipment, such as: printers, electric shavers, drills, blenders, and the like. Most of the already well-known prior art solutions for the structural matrix use bronze, copper, silver, and pure iron. There are used as solid lubricants: molybdenum disulfide (MoS2), silver (Ag), polytetrafluoroethylene (PTFE) and molybdenum diselenide (MoSe2). This type of self-lubricating bushing, mainly with bronze and copper matrix containing, as solid lubricant particles, graphite powder, selenium and molybdenum disulfide and low melting point metals, has been produced and used for decades in several engineering applications.
However, these pieces do not present high mechanical strength, as a function of its high volumetric content (from 25% to 40%) of solid lubricant particles, which results in a low degree of continuity of the matrix phase, which is the micro-structural element responsible for the mechanical strength of the piece. This high content of solid lubricant has been considered necessary for obtaining a low coefficient of friction in a situation in which both the mechanical properties of the metallic matrix (strength and hardness) and the micro-structural parameters, such as the size of the solid lubricant particles dispersed in the matrix and the average free path between these particles in the formed composite material, were not optimized. The high volumetric percentage of solid lubricant, which has an intrinsic low strength to shearing, does not contribute to the mechanical strength of the metallic matrix.
Moreover, the low hardness of the metallic matrix allows a gradual obstruction of the solid lubricant particles to occur on the contact surface of the sintered material or product. Thus, in order to maintain a sufficiently low coefficient of friction, there has been traditionally used a high volumetric percentage of solid lubricant in the composition of dry self-lubricating composite materials.
A partially differentiated and more developed scenario, as compared with that previously described, is disclosed in U.S. Pat. No. 6,890,368A, which proposes a self-lubricating composite material to be used at temperatures in the range between 300° C. and 600° C., with a sufficient traction resistance (σt≧400 MPa) and a coefficient of friction lower than 0.3. This document presents a solution for obtaining pieces or products of low coefficient of friction, sintered from a mixture of particulate material which forms a metallic structural matrix and including, as solid lubricant particles in its volume, mainly hexagonal boron nitride, graphite or a mixture thereof, and states that said material is adequate to be used at temperatures in the range between 300° C. and 600° C., with a sufficient traction resistance (σt≧400 MPa) and a coefficient of friction smaller than 0.3.
Nevertheless, pieces or products obtained from the consolidation of a powder mixture simultaneously presenting the structural matrix powders and the solid-lubricant powders, such as for example, hexagonal boron nitride and graphite, have low mechanical strength and structural fragility after sintering.
The deficiency cited above results from the inadequate dispersion, by shearing, of the solid lubricant 20 phase between the powder particles of the structural matrix 10, from the condition illustrated in FIG. 1A of the enclosed drawings, to the condition illustrated in FIG. 1B, during the steps of mixing and conforming (densification) the pieces or products to be produced. The solid lubricant 20 spreads, by shearing, between the particles of the structural matrix 10 phase, and tends to surround said particles during the mixing and conforming steps, such as by compaction, by powder pressing, powder rolling, powder extrusion, as well as by powder injection molding, which steps submit said solid lubricant to stresses which surpass its low shearing stress, as schematically illustrated in FIG. 1B of the enclosed drawings.
On the other hand, the presence of the solid-lubricant layer between the particles (of the powder) of the structural matrix, in the case of a solid lubricant that is soluble in the matrix, does not impair the formation of sintering necks between the particles of the metallic structural matrix of the composite. However, in this case, the solid lubricant, by being dissolved during the sintering of the piece, loses its lubricating function, since the solid lubricant phase disappears by dissolution in the matrix. In the case of a solid lubricant that is insoluble in the structural matrix, such as the hexagonal boron nitride, the layer 21 formed by shearing (see FIG. 1B) impairs the formation of metallic contacts between these particles which form the structural matrix of the composite during the sintering; this contributes to a reduction of the degree of continuity of the structural matrix 10 phase of the composite material, structurally fragilizing the material and the obtained products.
Due to the limitations mentioned above, a technical solution becomes necessary both to prevent the solubilization of the lubricants when soluble in the structural matrix and to regroup the non-soluble solid lubricant dispersed in the form of a layer 21 in the steps of mechanically homogenizing and of conforming (densification) the particulate material mixture, in discrete particles during the sintering.
A similar situation to that described above occurs upon mixing non-soluble solid lubricant particles with the structural matrix particles of the composite material, the solid lubricant 20 having a particle size much smaller than that of the particles of the material which forms the structural matrix 10 of the composite (see FIG. 2B of the enclosed drawings). In this case, the much finer particles of the solid lubricant 20 tend to form a relatively continuous layer 21 between the metallic powder particles of the structural matrix 10, even with no shearing stresses during the processing steps previous to the sintering. The almost continuous layer 21 of fine particulate material of the solid lubricant 20 impairs the sintering between the particles of the metallic structural matrix 10, structurally fragilizing the final piece. In cases of insoluble phases, a more adequate distribution is that in which the particles of the particulate material of the composite matrix and the particles of the solid lubricant to be dispersed in the matrix present a particle size with the same magnitude order (see FIG. 2A).
Since the metallic structural matrix 10 is the sole micro-structural element of the composition that confers mechanical strength to the composite material to be formed, the higher the degree of continuity of the metallic matrix of said composite, the higher will be the mechanical strength of the sintered article or piece produced with the material. In order to maintain the high degree of continuity of the metallic structural matrix of the dry self-lubricating sintered composite material, it is necessary, besides a low porosity, a low volumetric percentage of the solid lubricant phase, since said solid lubricant does not contribute to the mechanical strength of the material and, consequently, does not contribute to the mechanical strength of the sintered products.
Therefore, there is a need for a technical solution, both to prevent the solubilization of the lubricants when soluble in the matrix and to regroup the solid lubricant which, by shearing, during the steps of mechanically homogenizing and conforming (densification) of the mixture, resulted in a distribution in the form of layers 21 in the volume of the material, impairing the sintering and the degree of continuity of the structural matrix 10 of the composite. The solid lubricant 20 should be dispersed in the volume of the composite material in the form of discrete particles uniformly distributed, that is, with an average free path “λ” which is regular between the particles of the metallic structural matrix (see FIG. 3). This allows promoting greater lubrication efficiency and, at the same time, a higher degree of continuity of the composite matrix, guaranteeing a higher mechanical strength to the self-lubricating composite material formed during the sintering, as illustrated in FIG. 3.
The compositions prepared to generate self-lubricating composites which present, as the material to form the matrix, the metallic element iron or ferrous alloys and simultaneously have the graphite as a solid lubricant, result in a self-lubricating sintered composite material with a matrix which can be excessively hard and fragile and with a coefficient of friction above the expected and desired one, due to the solubilization of the carbon by the iron matrix.
At the high sintering temperatures (superior to 723° C.), the chemical element carbon of the graphite is solubilized in the cubic structure of centered faces of the iron (gamma iron) or of the austenitic ferrous alloy. Thus, the use of a solid lubricant containing graphite causes an undesired reaction of the carbon with the iron, during the sintering, from temperatures above 723° C., producing a piece with reduced or no self-lubricating property, since the whole or most of the carbon of the graphite ceases to operate as a solid lubricant, forming iron carbide.
Said document U.S. Pat. No. 6,890,368 presents a solution for a material provided to form a metallic matrix and in which, in order to prevent the interaction of the solid lubricant, defined by the graphite, with the particles of the ferrous structural matrix, it is provided the previous coating of the graphite particles with a metal which, during the high sintering temperatures, minimizes the possibility of interaction of the coated graphite with the ferrous structural matrix.
While the solution suggested in U.S. Pat. No. 6,890,368 solves the problem of loss of the graphite solid lubricant during sintering of the piece by coating the graphite, said coating prevents the graphite from spreading to form a layer on the work surface of the pieces when in service (when frictioned in relative movement), reducing the supply of solid lubricant and thus making the lubrication less efficient. Besides, solely coating the graphite does not solve the fragility problem of the metallic matrix when the solid lubricant contains hexagonal boron nitride, which can, by shearing, generate a film between the matrix particles during the steps of mechanical mixing in mills and conformation (densification). The fragility problem of the sintered piece, due to shearing of the solid lubricant of hexagonal boron nitride, is not discussed in said prior US document, although this document considers the compaction and pre-sintering as one of the possible techniques for molding the piece to be sintered containing said solid lubricant of low shearing stress.
Apart from the deficiencies mentioned above, said graphite coating solution has a high cost, as a function of the materials employed and of the need of previous metallization treatment of this solid lubricant.
Moreover, the matrix types, generally used until recently for manufacturing pieces or products in self-lubricating composite materials, do not present the hardness necessary to prevent the particles of the solid lubricant phase from being rapidly covered, by the matrix phase, due to plastic micro-deformation caused by the mechanical forces to which the work surface of the piece is submitted, impairing maintaining a tribolayer by the solid lubricant spreading on said work surface of the piece.
The metallic matrix of the material is required to be highly resistant to plastic deformation, in order to operate not only as a mechanical support with the necessary load capacity, but also to prevent the solid lubricant particles from being covered by plastic deformation of the structural matrix, upon operation of the pieces (when frictioned in relative movement), preventing the solid lubricant from spreading in the interface where the relative movement occurs between the pieces.